Cook pasta according to package directions; drain.
Meanwhile, for sauce, in a medium saucepan cook the mushrooms and onion in the margarine or butter until vegetables are tender. Stir in cream cheese, salt, and pepper. Cook and stir over low heat until cheese is melted. Gradually stir in milk and chives; heat through.
Pour sauce over pasta. Toss to coat. Serve immediately. Makes four side-dish servings.